This invention relates generally to kaolin clays, and more specifically relates to a process for beneficiating a kaolin clay to improve the rheological properties of a high solids suspension subsequently formed from the beneficiated clay, thereby rendering same more suitable for paper and paperboard coating applications.
Kaolin clay coating pigments having very fine particle size and high brightness characteristics, are widely utilized in the coating of merchant grade papers and various types of paperboard wherein high gloss and smoothness of coating is required. Typically, these pigments are applied as a high solids aqueous suspension, i.e., a suspension including from approximately 60-75% by weight of clay solids. The size distribution of prior art pigments used for such purposes are usually such that of the order of 90-100% by weight thereof are of less than 2 microns equivalent spherical diameter (E.S.D.). Typically, further, the brightness characteristics, as measured by the standard specification established by TAPPI procedure T-646m-54, are of the order of at least 90.
Among the further qualities of a high solids coating clay slurry, which are of paramount importance for achieving high quality coatings, are the viscosity characteristics of same. It may be noted in this connection that the term "viscosity" as used herein with respect to clay slurries, refers to such characteristics as determined by the procedures of TAPPI Method T 648 su-72, as revised in 1972. This method sets forth specific procedures for determination of both the "low shear" and "high shear" viscosity. The latter, i.e. the high shear viscosity, is considered of special importance in evaluating a high solids clay slurry for the aforementioned coating purposes.
While numerous products are known and commercially available which are quite adequate (and in many instances of excellent quality) with respect to brightness, and to a lesser extent, particle size distribution, the high shear rheological qualities of these coating clays are less than would be ideally sought for. Efforts have therefore been made over the years to improve the rheological properties of such clays; but by and large, little direct success has been achieved. In general, the rheological properties have been accepted as those which are inherent in the particular starting materials used.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method for beneficiating a kaolin clay to improve the high shear viscosity of a high solids suspension subsequently formed from the beneficiated pigment.
It is a further object of this invention to provide a method as aforesaid, which simultaneously improves the size characteristics of the clay, to markedly decrease the proportion of the clay particles which may have deleterious size or shape characteristics.